The present invention relates to a polysilicon resistor element and a semiconductor device using the same.
As SRAM is a typical example of a semiconductor device having a polysilicon resistor element. That is, a resistive load MOS SRAM includes a polysilicon resistor element as a load resistor for an inverter conforming a flip-flip of each memory cell. The polysilicon resistor element is connected between a drive transistor of the inverter and a voltage source wiring in a memory cell array (referred to as "in-cell voltage source wiring", hereinafter). The polysilicon resistor element and the in-cell voltage source wiring are formed from polysilicon films formed simultaneously. The polysilicon resistor element includes a resistor portion which is lightly doped with impurity and lead portions which are heavily doped, and the in-cell voltage source wiring is also heavily doped with impurity. That is, the lead portions of the polysilicon resistor element is electrically connected to the in-cell voltage source wiring.
A formation of such polysilicon resistor element and in-cell voltage source wiring will be described.
A semiconductor chip having drive transistors, etc., formed therein and covered with an interlayer insulating film is prepared and through-holes are formed in drain regions of the drive transistors. A non-doped polysilicon film having thickess in the order of 100 nm is deposited thereon by low pressure CVD. The whole surface of this wafer is ion-injected with phosphor at 1.times.10.sup.14 cm.sup.-2 resulting in a resistive polysilicon film lightly doped with N type impurity. The resistive polysilicon film thus formed is selectively covered with a silicon nitride film having thickness in the order of 100 nm and an area of the polysilicon film which is not covered with the silicon nitride film is non-injected with arsenic at about 1.times.10.sup.16 cm.sup.-2 and the wafer is heat treated for activation.
A conductive polysilicon film thus formed constitutes the lead portions of the polysilicon resistor element and the in-cell voltage source wiring.
Thereafter, formation of a second interlayer insulating film, deposition of a PSG (phospho-silicate glass) film, reflow processing and formation of an upper wiring are performed, resulting in a semiconductor device. In other words, since there are several heat treatments after the arsenic ion implantation, diffusion of impurity may occur by which the lead portions of the polysilicon resistor element is expanded and an effective length of the resistor portion (resistive polysilicon film) becomes shorter.
Such expansion of the conductive polysilicon film due to diffusion of impurity depends upon heating temperature, time and diffusion coefficient of impurity. The diffusion coefficient of arsenic in a polysilicon film is larger than in monocrystalline silicon and depends largely upon grain size of polysilicon film. The grain size of polysilicon film is hardly controlled. Thus, the effective length of the resistor portion varies with variation of fabricating conditions of the semiconductor device. The polysilicon resistor element is generally designed by taking such variation of effective length into consideration. In order to limit power consumption of the memory cell within a permissible range, it is usual to make the design size of the polysilicon resistor element larger. Therefore, this face makes a source of obstraction against the requirement of miniaturization of memory cell array. For these reasons, it is very difficult to use polysilicon resistor element in a SRAM having capacity of 4M bits or more unless such variation of effective length is minimized.
The in-cell voltage source wiring is connected to a metal wiring of aluminum or aluminum alloy at a constant interval, that is, for example, every 256 memory cells, and supplied with a source voltage through the metal wiring. In a connecting portion between the metal wiring and the in-cell voltage source wiring, there may be a reaction between aluminum and silicon, such as interdiffusion, by which ohmic contact is degraded. Such problems can be solved by providing a barrier metal therebetween, if complicated processes for provision of such barrier metal is allowed.
Although the problem of expansion of the lead portions and the problem of connecting portion between the metal wiring and the lead portions are described for the SRAM as example, they are common to semiconductor devices having polysilicon resistor elements.